In general, charge storage devices of all types such as a primary battery secondary battery capacitor, supercapacitor and fuel cell are manufactured in accordance with various product standards based on their material, size and type.
Rechargeable secondary batteries are classified further into lead-acid, nickel-cadmium, nickel-metalhydride, lithium ion and lithium polymer batteries according to their chemistry and material.
These secondary batteries are different in various aspects such as average discharge voltage, discharge voltage profile, internal resistance, limiting discharge current, temperature characteristics, charge cut-off voltage and the like.
In order to select a battery appropriate for the requirements of a particular application among various battery products, many kinds of factors are to be considered such as battery capacity, size, price of the product or the like including characteristics of the battery described above. As a reference for the choice of the battery for suitable application, it is required to use a method for quantitatively representing the performance of a battery.
For example, a battery used to drive a DC motor must have a drive voltage greater than the minimum operational voltage of the DC motor. A voltage drop caused by the internal resistance of the battery during operation has a very significant effect upon the operating time of the DC motor depending upon the size of the drive current.
For a battery used as a portable power source for a GSM or CDMA type cellular phone, precise analysis is needed for the characteristics of the battery in association with the discharge of periodic high current pulses. A rechargeable battery for an electric vehicle requiring instantaneous high power also has to be analyzed precisely particularly for a transient current response.
As a method to analyze the operational characteristics of a battery precisely, it is possible to examine and analyze the internal parameters related to the mechanism of the battery as well as general characteristics such as battery capacity, average discharge voltage, discharge voltage profile, internal resistance, temperature characteristic and charge cut-off voltage.
These internal parameters of a battery can be determined through measurement of the impedance characteristics of the battery at a wide range of frequency.
The present inventors have applied for a patent on an effective method for measuring and predicting battery capacity with the results of this study (Refer to. Korean patent application Nos. 22540 (1998) and 24134 (1998)).
The most effective method for selecting a battery for the appropriate purpose is to represent an application device including a specified battery as an equivalent circuit and then simulate it in a numerical manner.
This method can evaluate the efficiency and stability of the device by a virtual combination of a selected battery and an electric or electronic device as an electronic circuit using a computer.
For example, this type of numerical simulation of electric and electronic devices has been developed as a general computer program known as SPICE at California Berkely University in America and widely used in many research institutes and manufacturing companies.
A conventional equivalent circuit of battery is however oversimplified in construction and has been utilized for the simulation of devices rather than the battery itself.
The conventional equivalent circuit of battery is made up of a constant voltage source and serial resistors without consideration of the variation of discharge voltage over time. Thus the model works effectively only for a short period of discharge time under a DC current.
A method using the Peukert parameter disclosed in U.S. Pat. No. 4,952,862 is not appropriate to precisely describe the discharge characteristics of a battery since it employs a phenomenological model which simply approximates the discharge profile linearly at the initial stage of discharge and exponentially at the late stage of discharge.
A method for simulating a thermal battery as described in U.S. Pat. No. 5,428,560, which covers discharge voltage and internal resistance of the battery, is valid in a case of a long time DC discharge but cannot sufficiently describe the characteristics of the battery under transient conditions of discharge.
The non-linear circuit model of a battery suggested in the present invention is designed to calculate the voltage response of a battery with respect to a predetermined electrical load such as DC, AC, pulse or impedance, which implements a precise and effective simulation of the operational characteristics of electrical circuitry including a battery.